1. Field of the Invention
The present invention relates to a low pass filter, and more particularly relates to a low pass filter including two LC parallel resonators.
2. Description of the Related Art
An existing low pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2013-21449 is known. This known low pass filter includes first to third capacitors, a first inductor, and a second inductor. The first capacitor and the first inductor form a first LC parallel resonator, and the second capacitor and the second inductor form a second LC parallel resonator. The first LC parallel resonator and the second LC parallel resonator are connected in series with each other. In addition, the third capacitor is connected between ground and a portion between the first LC parallel resonator and the second parallel resonator.
However, in the low pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2013-21449, it is difficult to adjust the interval between two attenuation poles. FIG. 6 is a graph showing the results of simulation of bandpass characteristics |S21| and reflection characteristics |S11| of a low pass filter according to a comparative example having the same configuration as that of the low pass filter disclosed in Japanese Unexamined Patent Application Publication No. 2013-21449. In FIG. 6, the vertical axis indicates the bandpass characteristics and the reflection characteristics, and the horizontal axis indicates frequency.
In the low pass filter, as shown in FIG. 6, two attenuation poles are formed at frequencies higher than the pass band. In the low pass filter, there is a demand for obtaining desired bandpass characteristics by adjusting the interval between these attenuation poles. An example of a method for adjusting such an interval between attenuation poles is to change the structures of the first inductor and the second inductor to adjust the strength of magnetic coupling between the first inductor and the second inductor.
However, it is possible to improve the magnetic coupling between the first inductor conductor and the second inductor conductor by physically increasing the distance therebetween, but, when such a change is made, there is a possibility that the performance characteristics of the entire filter is deteriorated such that fine adjustments are not easily performed with such a method. In addition, as a result of changing the structures, due to deterioration of the Q value, desired attenuation characteristics cannot be obtained.